Semiconductor photonic devices are highly useful in a variety of applications ranging from monitoring, analyzing and imaging light to the generation of energy from solar radiation. Typically such devices comprise semiconductor photoresponsive components such as photodiodes, photoransistors or photoresistors integrated with semiconductor processing circuitry. Such devices can be used as photodetectors and solar cells.
Semiconductor photodetectors are widely used for the detection of light, both visible and infrared. They exploit the internal photoelectric effect, where electron-hole pairs are generated in a semiconductor by photon absorption and contribute to electrical conduction inside the device, leading to a corresponding current at the contacts of the detector. Such detectors are fabricated singly for monitoring, in linear arrays for spectroscopy, and in two-dimensional (2-D) arrays for imaging.
One approach to the fabrication of photonic devices is to integrate photoresponsive components, such as photodiodes, with components for processing circuitry, such as transistors and capacitors. Most commonly, the photoresponsive components and the processing components are formed in the same plane. In some variations, the processing components are formed in a semiconductor substrate and photoresponsive components are formed in a layer overlying the substrate. In such devices the photoresponsive components will have a finite thickness limited by several factors, including the time and conditions needed to deposit or grow the photoresponsive components, the height of metal contacts to the substrate, and the desired planarity of the overall integrated device. As a consequence, the photoresponsive components can absorb only a portion of the light falling on them, and some of the light can be transmitted through them without being absorbed. For light near the cutoff wavelength, photodetector absorption can be quite weak. For instance 20% might be reflected, 10% might be absorbed and 70% might be transmitted through the detector.
Light transmitted through the photoresponsive component can penetrate into the substrate, where it becomes stray light that can be absorbed (directly or after reflection) in the semiconductor circuitry, causing unexpected and undesired current to flow. Also, in a photodetector array, light that enters one photoresponsive component at an oblique angle may pass through it to another component in the array. Such stray light causes undesired crosstalk in an analyzer or blurring in an imager. Thus light that is not absorbed by the photoresponsive component can create three problems: it can degrade responsivity of the photoresponsive component, it can cause unwanted current to flow in the circuitry, and it can cause crosstalk. Accordingly there is a need for photonic devices that are configured to enhance responsivity while reducing stray light.